This invention is generally directed towards the cancellation of echos by a modem using a communication link such as a two wire telephone network. This invention more specifically addresses the problem of identifying and cancelling echos which are remote relative to a modem.
As used herein, an echo refers to the reflected portion of a transmitted signal which returns to the transmitting source or modem because of discontinuities or impedance mismatches in the communications link. Impedance mismatches may typically occur in a telephone system at a 2 wire to 4 wire transformation such as by a hybrid. Such echos represent an undesired or interfering signal to the receiver in the modem.
Echos are normally classified as near-end or far-end echos. A near-end echo is a reflection which normally occurs relatively close in distance and not delayed too long in time relative to the transmitting modem. A far-end echo refers to a reflection which is remote, usually in distance and time, to the transmitting modem and may typically occur near the remote modem.
Determining the exact location (time delay from the origination of the signal to the received corresponding reflection) of a remote echo makes it easier to achieve a high degree of cancellation of the echo. Existing modem systems assume that the most significant remote echo occurs at the remote modem site and uses the time it takes a signal to travel from the local modem to the remote modem and back to the local modem to calculate the time interval to be utilized for remote echo cancellation. This technique has several problems. First, this method requires that the remote modem be active in order to originate a signal to be transmitted back to the local modem. This method also depends upon the consistency in the detection of the signals by the remote modem and the consistency in the transmission of a corresponding signal by the remote modem to the local modem. If an echo originates at other than the remote modem site, this method is completely inadequate in order to determine the location of such an intermediate echo. Because of signal detection variations, this prior method cannot provide the exact location of an echo occurring at the remote modem site; accuracy of plus or minus 4 baud or more is typical for such systems.